The present invention relates to a thin film transistor with the use of a semiconductor film made of an aggregate of organic semiconductor molecules, and a manufacturing method therefor, particularly the thin film transistor characterized by having a controlled pattern shape of an electrode by improving an orientation order of the organic semiconductor molecules making up a semiconductor film with the use of a selectively disposed self assembled monolayer film, a manufacturing method therefor, and a visual display unit using it.
Recently, an active matrix type display unit using an active device represented by a thin film transistor (TFT) expands its market, while being used for not only a notebook-sized personal computer or a mobile telephone but also a monitor of a workstation or television, by making use of advantages of the excellent picture equivalent to that of a Cathode-Ray Tube (CRT), a low power consumption and a saved space compared to the CRT. However, the active matrix type display unit is more expensive than the CRT, and it is necessary to further lower the price for wider spreading. As one of methods for lowering the price, an application of the organic thin-film transistor (TFT) which can be manufactured by a simple and easy method to the active device, is under examination. A plasma chemical vapor-deposition (CVD) apparatus used for manufacturing an insulation film or a semiconductor film of a current amorphous silicon TFT, and a sputtering apparatus used for manufacturing an electrode, are expensive. The CVD process has a low throughput, because of requiring a high temperature of 250–300° C. for forming the film and a long time for maintenance such as cleaning. A vacuum evaporation apparatus used for manufacturing the organic TFT or a coating device is more inexpensive than the CVD apparatus and the sputtering apparatus, has the low film-forming temperature, and is easy for maintenance. Accordingly, the application of the organic TFT to the active matrix liquid-crystal display may promise a significant cost reduction.
A typical organic TFT is made of a substrate, a gate electrode, an insulation film, a drain electrode, a source electrode, and an organic semiconductor film; and performs a switching operation by modulating an amount of carriers accumulated in an interface between the insulation film and the organic semiconductor film from an accumulation state to a depletion state, with applied voltage to the gate electrode, to change an amount of the current passing between the drain electrode and the source electrode. The organic semiconductor film consists of an aggregate of the organic semiconductor molecules consisting of low-molecules or macromolecules. Known monomeric materials include an acene-based material represented by pentacene or thiophene oligomer, while polymeric materials include poly-3 and hexyl thiophenes (P3HT) which belong to polythiophenes and have a regioregular (having such an array as the whole chain forms a line in a same direction and the head and the tail are connected) structure of a highly regular nanostructure; a copolymer of fluorene-bi-thiophene (F8T2) as a polyfluorene system; and polyphenylene vinylene (PPV).
In order to obtain a high switching operation by using the organic semiconductor films, the organic semiconductor film formed on the surface of the insulation film needs to have all the organic semiconductor molecules oriented in the same direction and disposed densely.
As a method for forming the organic semiconductor film having the highly oriented order, a method is known in which the surface of the insulation film is previously coated with a self assembled monolayer film (SAM). For instance, as described in Applied Physics Letters 81(23), pp. 4383–4385 (2002) by A. Salleo and others, when the surface of the insulation film comprising a thermally oxidized film of silicon is coated with a SAM such as octadecyl trichlorosilane (OTS), the orientation order of the organic semiconductor film consisting of the F8T2 semiconductor macromolecules is improved, which leads to improvement of the switching performance. In addition, as described in IEEE Trans. Electron. Devices, 44, pp. 1325–1331 (1997), by Y. Y. Lin and others, it is reported that when the surface of the insulation film comprising the thermally oxidized film of silicon is coated with OTS, the orientation order of the organic semiconductor film consisting of the semiconductor low molecules of pentacene deposited by vacuum evaporation is improved, and crystal grain sizes of the organic semiconductor film deposited on the OTS coated thermal oxide film is greater than those deposited on the thermal oxide film without OTS coating, which leads to improvement of the switching performance of the TFT. In addition, as described in SCIENCE Vol. 280, pp. 1741–1743 (1998) by H. Sirringhaus and others, it is known that the electric field-effect mobility of the formed organic semiconductor film is improved to 0.01–0.1 cm2/Vs, when the surface of the insulation film is previously coated with a SAM consisting of hexamethyldisilazane, and then with P3HT thereon.
As described above, it is reported that a method for forming an organic semiconductor film on the surface of an insulation film previously modified with SAM, improves an orientation order of the organic semiconductor film and a switching performance, and provides an organic TFT having a performance equal to or better than that of a current TFT which uses an inorganic semiconductor of amorphous silicon for the semiconductor film.
When the thin film transistor is used in a display device using a backlight, such as a liquid crystal display, a light leakage current caused by photoirradiation from the back side of a substrate should be small to show an adequate switching performance as the thin film transistor. The light leakage current is generated because the semiconductor film absorbs the light applied from the back side of the substrate in a state that a voltage causing the depletion of carriers in the interface between the semiconductor film and the insulation film is applied to the gate electrode, and forms photocarriers which travel due to the voltage applied to drain/source electrodes. It is known that the amount of the leak current is large, when a travelling property of the carriers is high in the semiconductor film formed in the region outside the gate electrode-projected region of the surface of the insulation film, which is not shaded by the gate electrode, and directly absorbs the light applied from the back side of the substrate, particularly in a potential floating region which is not included even in the drain/source electrode region. It is described in detail, for instance, in Display and Imaging Vol. 7, pp. 129–135 (1998) by Wakagi and others.